Not applicable.
Not applicable.
1. Field of the Invention
The present invention relates to particle beam devices with ions or electrons, and more particularly to such devices that are used for imaging, analyzing, and processing of specimen surfaces.
For image production a focused particle beam is guided in a raster over the specimen surface and the signals which result from interaction with the primary particles are sensed and associated with their respective place of origin. Besides a high spatial resolution which is attained by a small diameter of the particle beam in the plane of the specimen, as high as possible an efficiency of the detection of the different signals is required.
2. Discussion of Relevant Art
The resolution of particle beam devices is in principle at its best when the specimen is situated very close to the objective lens or even inside this lens. This has as a consequence that the detection system for the detection of secondary particles and/or back-scattered particles must be arranged inside the objective lens or in a region which is free from fields and is between the objective lens and the particle source. The best resolution, particularly at low particle energies, is attained with arrangements in which the particles in the beam production system are first accelerated to a high energy and are braked to the desired energy at the end of the particle optical arrangement in an objective lens or in the region between the objective lens and the specimen.
Corresponding particle beam devices are described in, for example, U.S. Pat. No. 4,831,266 and U.S. Pat. No. 4,926,054. A very high resolution is attained even at low particle energies by means of the combination of an electrostatic lens and a magnetic field in the objective lens. Furthermore, the particles emitted by the specimen by back-scattering or in other ways are accelerated in the opposite direction by the electrostatic field and are imaged on an annular scintillation detector above the objective lens. In U.S. Pat. No. 4,896,036, a similar arrangement is described, likewise with an annular detector, but the objective lens is of course a purely electrostatic lens.
In these known systems, it has been found to be disadvantageous that the annular detector with scintillator and glass light conductor has to have a relatively large aperture of about 2-3 mm so that the primary particle beam is not obstructed by the annular detector. Computations and experiments have shown that up to about 80% of the particles arising at the specimen pass through the central opening of the detector and are therefore not detected. The detected signal is therefore very weak. Furthermore, the detected particles are not sensed integrally, and consequently cannot be separated according to energy and starting angle.
It is known from an article in Nuclear Instruments and Methods in Physics Research A, Vol. 363, pp. 31-42 (1995) to deflect, in regions far from the axis, particles emitted from the specimen or back-scattered at the specimen, without appreciably affecting the primary particle beam, by a suitable arrangement of two Wien filters. This means indeed leads to an improvement of the detector efficiency; here also, however, a discrimination of the detected signals according to starting angle and the like is not possible.
A raster electron microscope is described in U.S. Pat. No. 5,644,132, in which the annular detector has several annular divisions. Electrons back-scattered at the specimen, which are more strongly detectable in the region near the axis than are secondary electrons, are to be separated from the secondary electrons which mainly are far from the axis by this division. In connection with an embodiment example, the possibility is also mentioned that for reasons of mounting, the inner annular detector and the outer annular detector can be arranged to be slightly offset in the direction of the optical axis. Such an annular division of the detector indeed basically permits a separation of the detected electrons according to their starting angle on emergence from the specimen. The problem that a large fraction of the secondary electrons and of the back-scattered electrons is transmitted through the central bore region and consequently not detected at all cannot be solved by this annular division of the detector, however.
The present invention has as its object to improve the detection in a particle beam device of the secondary particles emitted by the specimen and the particles which are back-scattered at the specimen. Furthermore, a selection or coordination of the detected particles according to their starting angle is to be possible.
This object is attained according to the invention by a particle beam device comprising a beam producer, an objective that focuses a particle beam onto a specimen, and two detectors arranged between the beam producer and the focal plane of the objective for particles back-scattered and emitted from the object. The two detectors are arranged offset from each other in the direction of the optical axis, and the distance between the two detectors is at least 25% of the distance between an object-side detector of the two detectors and the focal plane of the objective. Advantageous embodiments of the invention will become apparent from the features described in the specification and set forth in the claims.
In the particle beam device according to the invention, two detectors for the particles back-scattered from the object or for the particles emitted from the object are arranged, mutually offset in the direction of the optical axis of the particle beam device. The spacing between the two detectors in the direction of the optical axis amounts to at least 25% of the distance between the object-side detector and the focal plane of the objective, which focuses the particle beam on the specimen. The specimen-side detector then serves for the detection of those particles which emerge from the specimen at a relatively large solid angle, while the source-side detector serves for the detection of those particles which emerge from the specimen at a relatively small solid angle, and which are transmitted through the opening provided through the specimen-side detector for the passage of the primary particle beam through it. By means of the axially offset arrangement of the two detectors, even the particles transmitted through the central opening of the object-side detector can be detected with the source-side detector, when both detectors have central openings of the same diameter. Preferably, however, the central opening for the passage of the primary particle beam at the source-side detector should amount to at most a third of the diameter of the central bore of the object-side detector. Correspondingly small bore diameters, of less than 0.2 mm for example, are then possible through the source-side detector, when this detector is embodied as a conversion diaphragm, at which secondary electrons again arise on to the impingement of high energy particles themselves. The secondary electrons emerging from the conversion diaphragm are then detected by a laterally arranged conventional detector, which produces an electrostatic extraction field for the secondary electrons emerging from the conversion diaphragm. The material of the conversion diaphragm should be a relatively light element with an atomic number xe2x89xa620, e.g., aluminum or carbon, since such light elements have a relatively high secondary electron yield.
As in the prior art mentioned at the beginning, both detectors have an annular detection surface and are symmetrical with respect to the optical axis. In contrast to the prior art mentioned at the beginning, however, the external diameter of the detection surface of the source-side detector is greater than the diameter of the central bore of the specimen-side detector.
Insofar as the source-side detector has only a very small bore bounding the aperture of the primary beam for the primary beam to pass through, this detector is to be received on an adjusting device which permits an adjustment of the detector in the two directions perpendicular to the optical axis.
For the evaluation of the signals detected with the two detectors, a signal processing electronics can be provided which produces output signals from the detector signals corresponding to linear combinations of both detector signals. By the formation of corresponding linear combinations, the effects of the surface topography of the specimen can be amplified, and charge contrast images can be produced. This is advantageous in particular when thin layers with conductivity different from the matrix are investigated in the particle beam device. The coefficients of the linear combination are then to be freely selectable by the user of the particle beam device.